Sparsha is the clearest single case for seeing why Samkhya's account of the five elements is additive rather than substitutive. If each tanmatra simply replaced the one before it, vayu would carry only touch and akasha would be left carrying nothing once air appeared; instead, the classical model has each new element inherit the full set of qualities established so far and add one further quality of its own, so vayu carries shabda and sparsha both, tejas will carry shabda, sparsha and rupa, and so on down to prithvi, which carries all five.
The reasoning behind this accumulation is that grossening is a process of increasing differentiation and density, not a process of one thing turning into an unrelated other. Sparsha does not erase shabda; it is added atop a quality that persists, in the same way ahamkara does not erase mahat's discernment when it produces its own further evolutes, and prakriti does not stop existing once mahat, ahamkara and everything downstream have evolved from it. Continuity across the sequence is the whole point of satkaryavada, Samkhya's doctrine that the effect pre-exists in its cause and evolution is transformation rather than creation from nothing.
Sparsha's specific contribution, tactile quality, is therefore best understood not as air's defining trait in isolation but as the second layer in an accumulating stack, one that vayu carries alongside sound and that every grosser element after vayu, tejas, apas and prithvi, will go on carrying in turn, alongside whatever further quality each of them contributes.